Process for nitriding metals



Feb. 20, 1962 J. MULLER EI'AL 3,022,204

PROCESS FOR NITRIDING METALS Filed March 20, 1961 2 Sheets-Sheet 1 IO 8 3 g; 9-

1g a aerated l 3:? g 1. 5 o A O 6- "6 a not aerated 2 4 8 O E 3 WEAR RESISTANCE O. I O

I l I lo KCNO Concentration of Cyanate 8 aerated I c 7 E .2 I E 6 J i O 5 4 FATIGUE RESISTANCE "6 3 i E 2 S z I C mud I I I I I l I I 20 3'0 40 I. KCNO- Concentration of Cyanate JNVE IYTORS JOHANNES MULLER BY CARL ALBRECHT Wwi ATTORNEYS Feb. 20, 1962 MULLER ETAL 3,022,204

PROCESS FOR NITRIDING METALS Filed March 20, 1961 2 Sheets-Sheet 2 L 4 s V V \1, UN V J U JNVNTORS JOHANNES MULLER BY CARL ALBRECHT ATTORNEYS il'nited States Patent 3,022,204 PROCESS FOR NITRIDING METALS Johannes Miiller, Frankfurt am Main, and Carl Albrecht,

Kronbert (Taunus), Germany, assignors to Kolene Corporation, Detroit, Mich.

Filed Mar. 20, 1961, Ser. No. 97,060 18 Claims. (Cl. 148-155) This invention relates to an improved nitriding process for imparting increased wear and fatigue resistance to nitridable metals, particularly ferrous metals, to improved nitriding baths for treatment of such metals controlled by aeration during the treatment; to improved apparatus useful in such treatment; and to the improved nitrided metals, particularly ferrous metals.

The invention is a continuation-in-part of our copending applications, Ser. No. 17,541, filed March 25, 1960, and Ser. No. 40,555, filed July 5, 1960, each now abandoned.

According to the invention, nitridable metals, particularly ferrous metals, are nitrided by immersing in a specific molten salt nitriding bath maintained by aeration. The nitriding bath comprises at least 25% cyanate by weight, usually adjusted in use to the range of 30 to 40% cyanate, and preferably the cyanate content of the bath is maintained in the range of v32 to 38%. The cyanate salt may be of any alkali metal, but its quantity is usually calculated as the potassium salt. The bath further contains from 40 to 75% cyanide, preferably 50 to 60% cyanide, which too may be of any alkali metal, but its quantity usually is calculated as the sodium salt. In use, the air introduced into the bath combines with the released carbon to form alkali carbonate, so the remainder of the bath, up to 25%, may be carbonate; and sometimes it is preferred to add the carbonate from the start to avoid substantial variation in composition of the bath in use.

When a nitridable'metal such as a ferrous metal is immersed in said bath its effect may be catalytic to release nitrogen and carbon in a certain ratio; at least from the salt layer in contact therewith, and at the critical temperature in the range of 500 to 600 C. The carbon being less soluble, deposits in the surface of the metal forming ferrous carbide, Fe C, and the nitrogen may be absorbed by the metal indefinitely, except that the precipitated carbide tends to precipitate the nitrogen as lower ferrous nitrides revealed by analysis as comprising FCgN in proportions of about 6.7% of the outer surface compound layer, the balance of the surface compound layer being ferrous carbide, Fe C. That surface layer is free of the usual brittle higher ferrous nitride, the presence of the latter being Fe N, typical of common nitriding treatments heretofore practiced in this art. I

This so-called surface compound layer extends for a considerable depth in from the outer surface of from about 4 to 12 microns, more usually 7 to 10 microns, such surface compound layer being considerably deeper than the usual 2 to 3 micron layer of Fe N present in ferrous metals nitrided under usual nitriding conditions in usual cyanide nitriding baths.

The outer compound layer of this invention, as described, is quite wear resistant, but is tough and ductile and does not materially increase the normal hardness of the ferrous metal. Beneath the outer layer, Fe N is also formed in a diflusion zone extending into the metal by further absorption of nitrogen for a considerable depth .up to about 0.3 to 0.4 mm. Thus, the compound layer is generally deeper and the diffusion zone is considerably more shallow than in usual ferrous metal nitriding penetrations. The outer compound layer imparts greater wear resistance and the absorption zone imparts greater fatiguestrength to the metal, both markedly in contrast to other nitrided ferrous metals. It is characterized as 'ice indicated by a substantial content of both Fe N and possibly Fe N and by substantially complete absence of Fe N. The average overall nitrogen content of the absorption zone exceeds about .0l% of dissolved nitrogen. Nevertheless, despite the usual increase in Wear resistance and fatigue strength of the metal nitrided according to this invention, the unusual great increase of surface hardness attending usual nitriding metal in a cyanide bath does not take place.

While all ferrous metals are improved by the compound layer and diffusion layers formed in the nitriding treatment of this invention, obviously some metals will show more marked improvement in wear resistance and fatigue strength than others; that is, metals which do not already have a considerable strength in these factors will be more markedly improved than other ferrous metals. In this respect the unalloyed ferrous metals, low and medium carbon steels or low alloy ferrous metals show the greatest improvement. However, improvement is evident in medium alloy steels as well as high alloys such as stainless steels. Thus, according to the invention, any steel such as unalloyed steel, low alloy steel, low and me dium carbon steels, medium alloy steel, stainless steel and gray cast iron are usefully treated to impart improved fatigue strength and wear resistance according to this invention.

As is known, alkali cyanate salt decomposes with release cf oxygen at high temperatures and cyanide absorbs oxygen at low temperatures to form alkali cyanate. Thus; any typical nitriding bath comprising alkali cyanide would absorb some oxygen from the air to give a content up 7 to 5%, and in some instances, even as high as 15% cyanate results from usual operation. For these reasons, cyanate and cyanides for nitriding purposes have often been referred to interchangeablybecause cyanides absorbing oxygen from the atmosphere include some cyanate and cyanate decomposing at high temperatures will form cyanide.

The critically improved range of 25 to 40%, and preferably 30 to 40% cyanate content of the bath, as stated above, not only distinguishes such art, but is maintained at a temperature of 500 to 600 C. and the metal is nitrided in such bath under these conditions usually by immersion of the metal therein for a period up to two hours or more. Some metal improving effect takes place almost immediately upon immersion. immersion for.pe-. riods longer than two hours does-not appear to result in further substantial improvement effects. Shorter immersion periods, as little as a few minutes, will show some improvement. It is preferred to immerse the metal for a period of 30 to 120 minutes, a most usual period being about to minutes.

Such immersion of the metal in a controlled bath of the stated composition will improve the metal bath as to hard wearing surface strength as well as fatigue strength in recognizable contrast to other nitriding procedures, particularly in baths of different compositions. Nevertheless, the metal, bath and metal treating procedure of this invention is most critically further improved by aerating the bath during the immersion therein of the metal by introducing a gas-containing oxygen, usually ordinary air. Such aeration is preferably as small, finely divided, evenly distributed bubbles emitted from a perforated or porous tube mounted as an annular ring in or near the bottom of the bath so that the bubbles flow upward through the bath. That continued aeration very sub st'antially further improves the effect of .the bath treatment upon the metal both as to its wear resistance and fatigue strength.

Such improvemens are illustrated in the drawings whereinvFlG. 1 plots as ordinate the depth of the com- 'hibited in FIGS. 1 and 2 curves.

little or no aeration.

have a bath of substantial depth as well as diameter and aeeaaea pound layer against the concentrations of cyanate in the bath as abscissa. It will be observed in curve A that the depth of the compound layer begins noticeably to increase at about 25% cyanate content of the bath, even where the bath was not aerated. Aeration of the bath, however, produces most contrasting effects as shown in curve B, particularly in' the depth of the compound layer and as will appear the so-treated metal product has much greater wear resistance inrterms of compound layer thickness with respect to the unaerated bath treatment.

Again, FIG. 2 compares in curve C the effect upon the fatigue strength of the treated metal, plotting the number of cycles until fracture of an impressed load of 71,160 I last in millions of stress cycles as ordinate against the cyanate content of the bath. It will be noted again that the metal is improved beginning at about 25% in any treatment, the improvement being most marked at 30%. However, the fatigue strength of the metal treated in the unaer-ated bath, curve C, is contrastingly inferior to the fatigue strength of metal treated in the same bath which has been aerated during the treatment.

'. Some aeration of the bath takes place naturally by absorption of oxygen from the air so that baths that comprise large area quite shallow pans will slightly exhibit better effects upon the metal treated therein than a bath which is deep, apparently because of some absorbtion of air into the bath by normal exposure. Deep baths wherein no substantial aeration of the bath salts is present, show immediate contrastingly poor results as ex- Even when substantial use is made of the shallow bath and many metal Workpieces are treated therein under continuous use periods, it too will exhibit the same relatively poorer results of For that reason it is preferred to to introduce the air into the bath at a controlled rate. The rateof introduction of the air providing substantial aeration as effected is not, however, critical. Considerable variation is therefore possible. An empirical formula for determining the rate of aeration of a bath of various dimensions in which; the rate of air introduction is correlated with such bath dimensions. Thus, while the aeration-of the bath can be varied beyond the limits given in the formula below, it is: preferred to stay within these limits for optimum operation.

up to 0.1, (1

It is found, surprisingly, that the aeration of the bath tends to invert the cyanate tosisocyanate substantially leaving an n-cyanate content of from 0.5 to about and preferably the n-cyanate content is about 1 to 4% of the total cyanate present in the bath. That iso-cyanaten-cyanate mixture with cyanide allows slight modification of the operative range of the total cyanate content to to 40% of the bath and the cyanide 40 to 60% of the bath, the remainder being carbonate. As above,

' the preferred-range is 32 to 38% cyanate, at least 40% cyanide, and the remainder carbonate.

Preferably thecyanide contents of the fused baths as used according to the invention is maintained at about 50 to- 60% calculated asNaCN. Also, the potassium content of the fused bath according to the invention, calculated as pure potassium, is between 10' and 30%, prefer ably about 18%. The potassium can be bound as the cyanate and/or cyanide. Sodium is essentially the remaining metallic component of the salts of the fused salt bath. If desired, alkali metal chlorides may also be inchided. in. the baths. according to. the invention.

The temperature at; which the fused salt baths according; to the invention areused can bebe tween 500 and 600 C.., preferably between about 540 and 570 C. V

The nitriding treatment according to the invention can he. applied: generally to metals generally capable of being or medium alloyed steels, 18-8 type and other stainless steels, cast irons and the like. a V

The process according to the invention not orly causes a substantial increase in thickness of the so-called compound layer, consisting of nitrides and carbides, formed at the surface of the metal, such as steel, treated, but also short ns the time required for the treatment and above all produces an extraordinary increase in the fatigue strength of the treated material.

To illustrate the effectiveness of the process according to the invention a series of tests were cairied out on samples of plain carbon steel (0.15% C.) using salt baths with varying n-cyanate content. The samples Were all treated in the salt baths for 90 minutes at 570 C.

The results attained in such costs are shown in the following table wherein the thickness of the compound layer Y2 which is composed of intermetallic compounds of Fe, C and N, the depth of nitriding NT, the fatigue strength WB and the content N of n-cyanatebased upon the total cyanate content of the various baths are given:

Bath YZ N'r, mm WB N, percent as 0. 2s '50, 000 0. l 10 0. as 2. 7x10 0. 9 12 '0. 40 4. 0x10 3. 7

breaking to withstanding 2.7 x10 changes in load. At the same time the thickness of the compound layer produced is increased several fold so. that the time required, for the treatment can be reduced substantially under that required for baths not containing the quantity of n-cyanate employed according to the invention.

The n-cyana-te content of a salt. mixture can easily be ascertained byultrared spectroscopy;

A proper n-cyanate is provided and maintained in the salt bath by passing air or other finely divided oxidizing gases through the fused salt bath at temperatures between 500 and 600 C. In another method the n-cyanate can be produced-independant from the salt -bath--by oxidizing alkali metal cyanides dissolved in water with H 9;

or permanganate at a temperature of 20 C. The extracted n-cyanate is then added to the salt mixture of. thel'bath.

There are several ways to maintain the critical bath composition as given. That-is, essentially, the critical content of cyanate. The simplest, of course, is merely to melt up a mixture of alkali metal cyanate, preferably potassium cyanate and alkali metal cyanide, preferably sodium carbonate up to about 25 %,.as would be formed in continuous use of the bath, the melt being made and maintained in the temperature range of 500 to 600 F. It is often desirable, and the practice is conventionally followed in the operation of this invention, to form the bath of cyanate -cyanide components which may have desnaaaoa parted from the preferred composition hereof and then adjust the cyanate-cyanide content of the bath after the melt is formed or after it has been used to bring it to the desired range by such adjustment. For example, a cyanate rich bath may be treated with a composition low in cyanate; that is, having from about to potassium cyanate, to potassium cyanide, 60 to 70% sodium cyanide. The alkaline metal chlorides when included in the bath are for purposes of modifying the fluidity of the bath. Thus, some adjustment of the bath composition can be made by overheating it; that is, temperatures about or above 700 C. to decompose more cyanate and thereby reduce its quantity increasing the cyanide whereby adjustment of the bath composition is effected merely by the raising of the temperature. It is preferred, however, to adjust the composition of the bath by adding to it mixtures of cyanate or cyanide rich or poor in the component needed to be increased or decreased.

As stated, the operation of the bath preferably is by aeration. During immersion, the bath in which the cyanate is largely isocyanate and contains 0.5 to 10% n-cyauate is much superior to an n-cyanate bath. Since that mixed cyanate bath may be formed by aeration, that aeration procedure is'the most economical to provide the mixed cyanate bath. However, a bath compn'singa 'salt mixture of the desired proportions of isocyanate,

normal cyanate can be formed by melting a mixture of these salts together with cyanide and if desired carbonate,

and maintaining the temperature at 500 to 600 C., preferably 540 to 570 C.

In a preferred operation of this invention the bath has its cyanate content maintained by maintaining the bath in a critical depth of about 0.5 to 3 meters and feeding by aerating the bath with air or other oxygen containing gas very evenly introduced, homogeneously in fine bubbles from the bottom of the bath upward whereby the air is evenly permeated and absorbed by the bath components at a fixed rate to maintain the cyanate concentration in the critical range.

The air or oxidizing gas is introduced into the bottom of the fused salt bath in such a way that it is finely distributed in the bath in the form of small bubbles which rise up through the molten bath. For example, the air can be introduced into the bath through one or more tubes lying upon the bottom and encircling the bottom edge or spiraling through the bottom to substantially cover the area thereof. The small perforations in the tubes are sized to emit a large number of very small bubbles. Instead of using such perforated tubes the air can also be introduced through porous ceramic or metal bodies to obtain the desired fine distribution thereof.

As during use of the salt baths according to the invention there is a tendency for the cyanate content thereof to build up, it is desirable to employ a mixture of 0-10% of KCNO, 20-30% of KCN and 6070% of NaCN or of 20-40% of KCN and 80-60% of NaCN as a make up addition to the bath after use. However, if desired, it is also possible to adjust the cyanate content of the used bath downward to maintain it in the desired range by superheating such bath between work loads to about 700 C. This not only causes a reduction in the cyanate content of the bath but also removes the ferrocyanides built up in the bath from the articles treated.

The temperatures at which the fused salt baths accord-- -ing to the invention are used can be between 500 and 600 C., preferably between about 540 and 570 C.

The nitriding treatment according to the invention can be applied to metals generally capable of being nitrided -and particularly the ferrous metals, for example, un-

cedure according to the invention above all ensures un'i-' form results with a good nitriding action and increases the wear resistance of steels, including stainless as well as cast irons, reduces seizing in case of deficient lubrication and elevated temperatures, improves corrosion resistance of non-alloy and low alloy steels and increases fatigue life of the treated workpiece.

While the process according to the invention generally produces an increase in the fatigue strength or life of the treated ferrous metals, it is preferable in the case of unalloyed carbon steels to quench the treated pieces in Water to keep the nitrogen in solid solution as the fatigue strength is thereby improved. With alloy steels, the fatigue strength is independent of the cooling rate after the nitriding treatment.

The nitrided parts produced according to the invention are characterized by no substantial increase in brittleness in contrast to the effect of the treatment of such metal where the nitrided surface contains substantial quantities of the hard brittle compound Fe N. While the salt bath nitriding process according to the invention does produce some slight increases in surface hardness, the surface-produced is characterized more by its toughness, wear resistance and anti-galling properties, than by its hardness.

In the prior patent of one of us, U.S. Patent 2,927,875, I have found the selenium and the telurium as well as sulfur increase the surface hardness. These elements are avoided in the present process where no substantial hardness increase takes place.

The salt bath used in the process according to the invention when, for example, operated at about 540-570" C. decomposes on contact with the parts to be treated. Specific amounts of carbon and nitrogen are liberated. The action of the nitrogen and carbon liberated can be described in the case of plain carbon steels which have ferrite as their main metallurgical constituent. Both carbon and nitrogen when subjected to the same conditions of dilfusion in ferrite have vastly difierent solubility rates. Carbon is ten times less soluble in .ferrite than nitrogen and therefore quickly forms iron carbide particules (F330) at the surface of the part. The iron carbides act as nuclei so that the nitrogen will precipitate at the surface to form the non-brittle and desirable iron nitrides diffusion zone can be proven not-only by chemical tests,

but also metallographically, if a sample is aged at 575 F. for one hour. Nitrogen is precipitated in the form of very conspicuous needles consisting of Fe N.

It should be mentioned that the Fe N or needle-like structure in the diffusion zone will not be visible metallographicallyif a part is quenched after the nitriding treatment. The part must be aged to develop this structure, but aglng is not required on production parts, and has no advantages.

. the invention will be found in all common constructional steels and in gray cast irons. and cast irons containlng alloys such as chromium, nickel,

On the other hand, steels etc., will produce compound layers as obtained in plain were quenched in water after the treatment. 7 oftests were carried out while passing 700- liters of finely v 7' carbon steels and grey cast irons. Compound layers of alloys steels and irons 'differonly slightly as their chemical composition is concerned. However, the effect in the difiusion zonecan be quite different from that described for plain carbon steels in that no needle-like structure is produced. This zone may be appreciably harder than in plain carbon steels andgrey iron.

On alloyed steels and iron nitrided according to the layer, regardless of the materiah'isin the range of approximately 550-750 Vickers.

The air or oxidizing gas is introduced into the bottom of the fused salt bath in such a way that it is finely distributed in the form of small bubbles which rise up through the molten bath. For example, the air can be introduced into the bath through a tube encircling the bottom edge of the bath provided with a large number of small openings. Instead of using such a perforated tube the aircan also be introduced through porous ceramic or metal bodies in order to attain the desired fine distribution thereof.

FIG. 3 diagrammatically shows an apparatus suitable for carrying out the process according to the invention in which the air is supplied to the bottom of the fused salt bath through a perforated annular tube encircling the bottom edge of the bath; and FIG. 4 diagrammatically shows the bottom portion of another form of apparatus in which the air is introduced through a porous bottom plate.

With referenceto FIG. 3, electric motor 1 serves to operate air compressor 2 which. supplies air to the bottom 'of salt bath 20 over valve 3, rotometer 4 (for measuring gas quantity), tubes 6 and 7 and perforated annular tube 8, provided with perforations 0.5mm. in diameter spaced about 20 mm. apart. A manometer 5 for measuring gas pressure is provided in the-air supply system after the rotometer. The salt bath furnace consists of steel pot 9 which serves to hold salt bath 20 and which is spaced from furnace shell 10. Shell 10 is lined with insulation 11 'of light porous insulating blocks which serves to support resistance heating coils 12. Thermo-couple 13 and temperature regular 14 are provided for controlling the temperature of the salt bath.

FIG..4 shows the bottom end of a pct 19 for the salt bath which in this instance is provided with a porous false bottom 18 through which the air supplied over tube V 17 is introduced into the salt bath instead of the perforated annular tube, as in FIG. 3.

The salt bath furnaces employed according to the invention preferably are well controlled as to temperature of operation so that temperatures more than 5 to 10 C. above that for which the control is set will not be reached. Too great a rise in temperature after the heating has been cut off by the regulator can be prevented, for example, by selecting an insulation material for the furnace which is not of the heat retention type so as to prevent'undue absorption of heat from the insulation after the regulator has cut ofi the heat. Furthermore, the heating provided for the furnace should be as uniform as possible to prevent local overheating which leads to cyanate losses. In addition, the heating provided should'be such that the length of time for the fused bath to reach the proper treating temperature after insertion of the work load is as short as practical and preferably less than about 20 minutes.

The following comparative tests will serve to illustrate V the improved nitriding action which is obtained according to the invention.

The tests were carried out in a fused salt bath 60 cm. in diameter and 100 cm. deepcomposed of 45% NaCN, 32% KCNO and 23% Na CO contained in a steel pot. The parts were treated for 90 minutes at 570 C. and all One series KCNO and the cyanide content of such bath is between before seizing occurred.

2) divided air per hour through the fused salt bath whereas the other series'was carried out without passage of air through the salt bath. In each series of tests sample parts of plain carbon steel (0.15% C.) were treated to determine the thickness of the compound layer and the diffusion zone achieved by the nitriding, test specimens of plain carbon steel (0.15% C.) for determining the fatigue life .weretreated, a specimen of a. chromium alloy steel (0.35% C, 1% Cr and 0.7% Mn) for determining the hardness'achieved on nitriding, and a specimen for determining the wear resistance achieved in nitriding in terms of the load applied before seizing occurs.

The following results were obtained:

With Aeration Without Aeration Thickness of the com- 10 microns 2 microns.

pound layer. Needle depth (depth of 039mm 0.23 mm.

nitrlding). Vickers hardness HVI 550 lrgJmmF--. HVI 541 kgJmrn. Load applied to wear re- 350 kg kg.

sistauce test specimens Fatigue life number load changes before rupture.

not ruptured after 7.4 million load changes.

ruptured after 53,000

load changes.

After the tests were completed, the composition of the fused salt baths are again analyzed and found to contain 34% of cyanate calculatedv as KCNO and 42% of cyanide :calculated as NaCN.

applying an increasing load upon a rotating nitrided workpiece seated in a bearing until seizing occurred. A Faville-Levally testing machine was employed for such wear resistance tests. The fatigue life tests were carried out by'rotating the nitrided. specimens under a unidirectional 50 kg. load at right angles to the axis of rotation and. determining the number of revolutionsthe test specimen withstood before breaking.

We. claim: l. A process. comprising immersing a metal workpiece in a, molten alkali metal salt bath comprising between about 25 to 40% cyanate, at least about 40% cyanide, the remainder being substantially carbonate, the said bath being free of sulfur, selenium and tellurium, while aerating the bath with an oxygen-containing gas introi duced in well-distributed fine bubbles.

2. The method of claim 1 in which the cyanate content of said bath is between 32 and 38% calculated as KCNO and the cyanide content of such bath is between 50 and 60% calculated as NaCN. V

3. The method of claim 1 in which the cyanate con tent of said bath is between 32 and 38% calculated as 50 and 60% calculated as NaCN, said bath having a potassium content of 10 to 30% calculated as pure potassium, the remaining metallic component contained in said bath essentially being sodium.

4. The method of claim 1v in which the cyanate content of said bath is between 32 and 38% calculated as KCNO and the cyanide content of such bath is between 50 and" 60% calculated as NaCN, said bath having a potassium content of about,l8% calculated as pure potassium, the remaining metallic component contained in 1 said bath essentially being sodium.

5. The. method of claim 1 in which said. fused salt bath' is between 0.5 and 3- meters deep and LHC. quantity of air passed therethrough in liters per hour is in the range from 052(1 p to i h wherein r signifies the radius of the salt bath in cm. and d the depth thereof in cm.

6. A method according to claim 1 in which the initial mixture fused in the production of said fused salt bath is a mixture of 25 to 40% of l-UNO and 60 to 75% of NaCN.

7. A method according to claim 1 in which the efiiciency of said bath in continued use is maintained by replenishment with a mixture of up to KCNO, 20 to 30% KCN and 60 to 70% NaCN.

8. A process for increasing the fatigue strength and Wear resistance without substantially increasing surface hardness of a workpiece of ferrous metal, transforming the outer surface into an outer ductile compound layer and forming a nitrogen containing diffusion zone therebeneath, said layer being characterized by the presence of nitride components comprising Fe N, comprising immersing the metal in a molten alkali salt bath comprising from 25 to 40% alkali metal cyanate, at least about 50% alkali metal cyanide, the balance being substantially alkali metal carbonate, the said alkali metal salt bath being free of any hardening element of the group consisting of sulfur, selenium and tellurium, and aerating the hot molten salt bath with finely divided bubbles of air as an oxidizing agent introduced into the bath.

9. A process for increasing the fatigue strength of metal workpieces comprising immersing the ferrous metal in a fused alkali metal salt bath containing alkali metal cyanide and alkali metal cyanate, the cyanate being in the range of about 25 to 40% calculated as potassium cyanate, 0.5 to 10% of the cyanate present in said bath being in the form of n-cyanate, and the remainder of the cyanate, as isocyanate.

10. The method of claim 9 in which said fused salt bath contains 20 to 40% of cyanate calculated as potassium cyanate, 30 to 60% of cyanide calculated as sodium cyanide and the remainder essentially alkali metal carbonate.

11. The method of claim 10 in which about 1 to 4% of the cyanate present in the salt bath is in the form of ll't y l i 12. The process as defined claim 9 in which the metal is ferrous metal.

13. The process as defined metal is ferrous metal selected from the group consisting of unalloyed low carbon steel, medium carbon steel, low alloy steel, medium alloy steel, stainless steel and cast iron.

l4. The process of claim 9 in which the cyanate-isocyanate salt mixture is in the range of 32 to 38%, calculated as potassium cyanate, and 30 to cyanide, ca ulated as sodium cyanide, and the remainder is essentially alkali metal carbonate.

15. A salt mixture adapted for nitriding metals comprising 20 to 40% of alkali metal cyanate calculated as potassium cyanide, 30 to 60% of alkali metal cyanide calculated as sodium cyanide and any remainder essentially alkali metal carbonate, 0.5 to 10% of the cyanate in claim 9 in which the content being in the form of n-cyanate and the remainder in the form of iso-cyanate.

16. A fused salt bath comprising 20 to 40% of alkali metal cyanate calculated as potassium cyanate, 30 to 60% of alkali metal cyanide calculated as sodium cyanide and any remainder essentially alkali metal carbonate, 0.5 to 10% of the cyanate content being in the form of n-cyanate and the remainder in the form of iso-cyanate.

17. A process comprising immersing a metal workpiece in a molten alkali metal salt bath comprising between 25 and 40% alkali metal cyanate and 50 to alkali metal cyanide, the said bath being free of sulfur, selenium and tellurium, while aerating the bath with an oxygen-containing gas in well-distributed fine bubbles.

18. Process of forming a metal treating cyanate-cyanide bath comprising melting together 20 to 40% potassium cyanate, 40 to 75% sodium cyanide and the remainder alkali metal carbonate, and maintaining said bath in the range of 500 to 600 C. while passing finely divided air bubbles therethrough, whereby to convert the cyanate content to a mixture which is predominantly isocyanate, the remainder of the cyanate from about 0.5 to 10% being normal cyanate.

References Cited in the file of this patent The Iron Age, Apr. 15, 1943, pages 41 through 45. Materials and Methods, July 1947, pages 75 through 7. 

1. A PROCESS COMPRISING IMMERSING A METAL WORKPIECE IN A MOLTEN ALKALI METAL SALT BATH COMPRISING BETWEEN ABOUT 25 TO 40% CYANATE, AT LEAST ABOUT 40% CYANIDE, THE REMAINDER BEING SUBSTANTIALLY CARBONATE, THE SAID BATH BEING FREE OF SULFUR, SELENIUM AND TELLURIUM, WHILE 